From what I understand, there are three parts to the OSR:

1. People spend a lot of money on an edition, and get pissed off when the editions upgrade after only a few years - a trend that has accelerated since TSR was acquired by WotC/Hasbro. (The amount of rage I've seen against TSR from before it was purchased is pretty amusing since that happened 20+ years ago).

2. Older editions had much less rules. This could be good because it makes it easier to learn and play, and doesn't require stopping play to look up rules constantly - but it also meant a lot of house rules, which could be widely divergent from one house to the next. This also produces a nostalgic feeling "for the good old days". Compared with the Pathfinder book, which is like 600 pages.

3. Older editions played with a much different atmosphere. Without skills or feats, and generally with less magical gear. In those games, a PC was an adventurer - not necessarily a hero. I think that's a subtle but profound difference in how people experienced the game.

There's probably more to it, and most of what I'm saying here is my interpretation of anecdotes I've come across in only the last few months so probably needs a pretty hefty grain of salt.